1. Field of the Invention
The invention relates to multilayer films consisting of a single- or multilayer support film and a polyethylene or polyethylene copolymer composite film, which is joined to the support film by a lamination and whose layers are produced from polyethylene or polyethylene copolymers with mechanical shock resistances differing by at least 25 cN, and of which at least one polyethylene or polyethylene copolymer has a mechanical shock resistance greater than 100 cN, as measured by ASTM D 1709-62.
2. Description of the Prior Art
Multilayer films are known and used to a great extent, for example, in packaging foods. These films consist of a support film and a further single- or multilayer film, which is laminated to the support film. Usually, the support film is heat-resistant, stretched biaxially and, if necessary, lacquered. Preferably, it is produced from regenerated cellulose, polyester, polyamide, polypropylene or aluminum. It may also be formed from several individual films of the said materials. The support film forms the outer layer in the finished package and must therefore be readily printable.
The film, laminated on the support film, forms the inside of the finished package. This film or, in the case of a composite film, at least the layer, of which the inside of the package will later on consist, must be readily heat-sealable and, for reasons of good processability, have a low coefficient of friction so as to ensure good processability on packaging machines.
For various areas of application, this film construction has however not yet proven to be satisfactory, especially because the puncture resistance, particularly for vacuum packaging, is not yet adequate. Consequently, there may be serious failures, such as leaks which, for many foods, can lead to spoilage of the goods. It might seem obvious to compensate for the low puncture resistance by an increased film thickness. However, this is not possible because many packaging machines can process only films of a particular maximum thickness. In the case of coffee packaging machines, this maximum thickness is 110 .mu.m.
Multilayer films of the aforementioned kind have therefore already been produced from support films and coextruded polyethylene films, which are laminated on the support films. The coextruded polyethylene composite films used consist of two layers joined together by a melt bond, one of which usually consists of the copolymer of a polyethylene of high mechanical shock resistance, that is, great toughness, and the other of a normal polyethylene (homopolyethylene, LDPE, low density polyethylene). Either the polyethylene layer of high mechanical shock resistance or--and this is preferred--the LDPE layer is joined to the support film by a lamination. Although it was possible to increase the puncture resistance by means of this system, these multilayer films also have deficiencies, so that they cannot be used satisfactorily in all areas. It is a disadvantage of such a multilayer film that it is insufficiently flat at elevated processing temperatures, so that breakdowns occur frequently, especially with packaging machines operating at high speed. The causes of this are, above all, the different coefficients of thermal expansion of the two layers of the composite film and the different film thicknesses of the two layers of the composite film, so that these act like a bi-metal. This phenomenon occurs particularly strongly in a multilayer composite film, in which the polyethylene layer of high mechanical shock resistance lies between the support layer and the the LDPE layer. On the other hand, if the polyethylene layer of high mechanical shock resistance is used as lower layer, the LDPE layer being laminated to the support film, the rolling tendency of the multilayer film is admittedly reduced. However, because the surface of the polyethylene layer of high mechanical shock resistance is generally dull, there are processing difficulties. The addition of lubricants to the polyethylene, from which this layer is produced, is possible but only within limits, because lubricants migrate into the other layers and reduce the strength of the lamination.
Owing to the fact that at least one layer of the coextruded composite film consists of a polyethylene or a polyethylene copolymer with a mechanical shock resistance greater than 100 cN, the known film admittedly has a certain puncture resistance, which however is not yet adequate for many areas of application.
Laminated plastics, which consist of several "interlocked" plastic films, are moreover known from the German Offenlegungsschrift No. 1,479,440. Rigid or semirigid plastic objects are formed from the laminated plastic by molding. It is noted that the object has a greater strength than a similar object produced from a single film of comparable thickness. Moreover, express reference is made to the greater vertical load carrying capacity of containers.
Furthermore, composite films are known (German Offenlegungsschrift No. 1,966,466 and German Offenlegungsschrift No. 2,102,377), which are formed by folding together blown tubes with walls of at least two coextruded layers. In these composite films, the inner layers are sealed together or melt bonded. As a result of the rotation of the blown film die, the molecules are oriented in crosswise directions in the folded walls of the tube. Consequently, the strength of the composite film as a whole is more balanced and improved.
It is an object of the invention to provide a multilayer, composite film of the aforementioned kind, which comprises a support and a composite film laminated on this support, has high puncture resistance, lies flat even at elevated temperatures and, if necessary, can slide well over metal on the side of the composite film opposite the support film.
To accomplish this objective, the invention provides for a coextruded composite film which is laminated on a support film, wherein the composite film is formed from a folded, two- to five-walled coextruded blown tube, the inner layers of the composite layers of the composite film being heat bonded and the outer layers being joined to the inner layers by melt bonding.
The decisive importance of the inventive multilayer film lies in the symmetrical construction of the coextruded composite film, which is laminated to the support film. The symmetrical construction, relative to the asymmetrical construction of the previously used two-layer composite films, causes the films to lie significantly flatter during processing, even at higher temperature differences. The initially described bi-metal effect does not occur because the forces, which lead to the tendency of the film to roll to the one side or the other, have been mutually eliminated by the symmetrical construction. The puncture resistance is increased. The higher puncture resistance is explained partly by the heat-bonding of the inner layers B, B'.
The composite film can be produced simply by blowing a multiwall tube and folding the tube while heat-bonding the inner layers. Moreover, the surface of the outer walls forming the outer layers is fused with the surface of the inner walls of the blown tube, which form the inner layers of the composite film. Using heat and pressure, the inner layers are interlocked or heat-bonded by means of a pair of press rolls. As a result of the customary rotation of the blown film die during manufacture of the tube, the orientation of the molecules between folded walls is crossed. Consequently, the strengths in the machine and cross directions are equalized to some extent. This leads to a further increase in puncture resistance.
Compared to previously known film constructions, which consist, for example, of a support film with an heat-bonded single-layer polyethylene film, the advantage arises that, for the same thickness of composite films, a significantly higher puncture resistance is achieved or, for the same puncture resistance, the total thickness of the composite film can be kept significantly lower, as a result of which a considerable savings of material and great advantages on the processing machines are achieved.